(a) Field of the Invention
The invention relates to a friction material composition suitable for the production of friction materials such as disc pads or brake linings to be used to brake cars, railroad vehicles and various industrial machines. It also relates to a method of producing a friction material composition and to friction materials made from the friction material compositions.
(b) Description of the Related Art
In cars, railroad vehicles and various industrial machines, friction materials are used to brake them. Friction materials mainly used at present are non-asbestos disc pads, which, as disclosed in Japanese Patent Application Examined publication No. 59-4462 and Japanese Patent Application Non-examined Publication No. 6-184525, contain as reinforcing fibers mixtures of various fibers, for example, metal fiber such as steel fiber, brass fiber or copper fiber, organic fiber such as acrylic fiber, aramid fiber or phenolic fiber, and inorganic fiber such as rock wool, potassium titanate fiber, alumina.silica fiber or carbon fiber.
When dry-mixed, the conventional non-asbestos friction materials suffer from the defect that their components, particularly cashew dust, are apt to drop off. Cashew dust has a larger mean particle sizes of 30 to 400 xcexcm than other components, and it is apt to drop off. The dropping of cashew dust adversely affects the uniformity of friction materials in their friction property, abrasion resistance and mechanical properties. Further, if the aggregates of the dropped cashew dust are mixed into friction materials, cracks will start from the portions containing the aggregates.
A common means for solving the problems is to prevent the components from dropping off by coating cashew dust with cashew nut shell oil or linseed oil or by employing wet mixing techniques using organic solvents or rubber latex.
As to the former method, the coefficients of viscosity of cashew nut shell oil and linseed oil ranging from 0.1 to 3 Paxc2x7s (100 to 3,000 cP) are too low to form coating having adhesive power enough to disperse cashew dust sufficiently, and the duration of the adhesive power of the coating is only about three days. The latter method can achieve good dispersion, but involves problems in that drying is required to remove the solvents, which increases the time and cost of the production of powder mixtures and complicates the production steps.
An object of the invention is to provide a friction material composition which effectively prevents cashew dust from dropping off by using a coating material having adhesive power of long duration and can form friction materials free of the local aggregation of cashew dust.
Accordingly, the invention provides a friction material composition, comprising
(a) a fibrous material,
(b) a binder containing a liquid rubber and
(c) a friction regulator containing cashew dust, the cashew dust being coated with 3 to 17 parts by weight of the liquid rubber relative to 100 parts by weight of the cashew dust.
Another object of the invention is to provide a method of producing a friction material composition, which effectively prevents the dropping of cashew dust by using a coating material having adhesive power of long duration, and gives a friction material composition suited to the production of friction materials free of local aggregation of cashew dust.
Accordingly, the invention provides a method of producing a friction material composition comprising (a) a fibrous material, (b) a binder containing a liquid rubber and (c) a friction regulator containing cashew dust, comprising coating the cashew dust with the liquid rubber, adding the fibrous material, remaining binder other than the liquid rubber and remaining friction regulator other than the cashew dust to the cashew dust coated with the liquid rubber, and then mixing.
Another object of the invention is to provide friction materials which are free of local aggregation of cashew dust.
Accordingly, the invention provides a friction material produced by molding with heat under elevated pressure the friction material composition of the invention, and a friction material produced by molding with heat under elevated pressure the friction material composition produced by the method of the invention.